The present invention relates to ultrasound systems and, more particularly, to ultrasound imaging systems with transducers comprised of two-dimensional phased arrays. A major objective of the present invention is to provide a two-dimensional phased array ultrasound system with more practicable control of steering and focussing than has heretofore been provided.
Ultrasound imaging systems have proved to be valuable tools for diagnosis in medical applications, as well as for analyses in several non-medical applications. One of the more prominent uses of ultrasound is the monitoring of a fetus during pregnancy. Ultrasonic energy transmitted into a body causes negligible disturbance, while reflections of ultrasound off tissue boundaries can be detected to characterize the internal body structure.
A typical ultrasound imaging system comprises a base unit, a probe, and an interconnecting cable. The electronics module generates an electrical pulse which is conveyed via the cable to the probe and converted to an ultrasonic pulse by a transducer in the probe. When the probe is pressed against a body, the ultrasound pulse is transmitted into the body and is reflected to different degrees at tissue boundaries within the body. The reflections from the various tissue boundaries reach the transducer at different times, depending on their distances from the probe. Typically, the transducer converts the reflections to a time-varying electrical signal. This electrical signal is processed within the base unit to form a video representation of the body being imaged.
Relatively simple ultrasound systems are known which employ spherical or parabolic transducers to transmit and receive ultrasound signals. Generally, these transducers are fixed focus so that their focal range is limited by the depth of focus of the transducer. Small apertures are required to obtain a large depth of field, but are limited to relatively low signal gathering ability and thus limited sensitivity. Provisions are typically made to steer the transducer mechanically to obtain image information over a range of angles. Mechanical steering requires the installation of a bulky motor in the probe and can impose reliability problems.
It is theoretically possible to provide both greater range and high resolution by deforming a transducer to vary its focal length so that a high resolution image is obtained for each of many focal depths. Apparently, it has not been practical to achieve the desired focal length control by mechanically deforming a transducer. On the other hand, "electronic deformation" of phased array transducers, a technology derived from radar, has permitted high-resolution imaging without significant depth-of-field limitations.
Phased array transducers comprise multiple transducer elements arranged in annular, linear or planar arrays. By varying phases, such as by introducing time delays, between elements of an array one can vary the depth of focus dynamically. Thus, a large aperture array transducer can be used to obtain high-resolution imaging and its focal point can be moved to overcome the limitation of a shallow depth of field.
Annular arrays come closest to simulating a mechanically deformable single-element spherical transducer. An annular array comprises multiple annular transducer elements arranged coaxially. As reflections are received by each of the annular elements, each annular element generates a corresponding electrical signal. By controlling the relative delays introduced in these electrical signals, the focal depth of the annular array transducer can be controlled. As with a spherical single-element transducer, an annular array must be mechanically steered to obtain a two-dimensional ultrasound image. The requirement of mechanical steering limits the speed and reliability of the imaging system. In addition, the requirement of a motor and drive train within a probe add mass and bulk to the probe, which should be small and lightweight.
A one-dimensional, e.g., linear, phased array comprises a series of narrow transducer elements arranged side-by-side. By controlling the phasing and relative delays among the elements, such an array can be electronically steered and focused in a steering plane, e.g., which is the azimuthal plane where the linear elements extend vertically. Within this plane, an ultrasound beam is steered and focused to discriminate a desired target from adjacent objects. The elements of a one-dimensional array should be spaced at most 1/2 the wavelength of the ultrasound signal to avoid grating sidelobe responses which degrade image quality.
A major disadvantage of one-dimensional arrays is that electronic focusing orthogonal to the steering plane, e.g., in the elevation direction, is not provided and resolution is set by the aperture size of the fixed focus acoustical lens. Resolution and signal gathering ability is limited. The elevation plane can only be normal to the array.
Two-dimensional, e.g., planar, array transducers comprise a multitude of small-aperture elements arranged in a two-dimensional array. As with linear array transducers, both focal depth and steering can be effected electronically. In contrast to one-dimensional arrays, steering and focusing can be effected anywhere within a cone-shaped volume in front of the array. Resolution in the elevational direction is provided. Resolution and signal gathering ability are significantly enhanced relative to a one-dimensional array.
However, planar array transducers are not widely implemented due to the large number of separate signal channels, one for each transducer element, which must be processed. For example, given a 5 MHz ultrasound signal, a .lambda./2 spaced array with an aperture of 15 mm by 15 mm to provide a resolution of about 1.degree. would require 100.times.100=10,000 transducer elements. If configured in a conventional manner, 10,000 electronic transmitters, receivers, and interconnect cables would be required. A cable between the probe and the base unit would have to carry over 10,000 lines, which is impracticable. In addition, the cost of a system which such a high component count is prohibitive.
If it were possible to perform the signal processing in the probe itself, only a single signal line would be required from the probe and to the base unit. Power, ground and control lines would still be required, but the number of lines required to be carried by the cable would be greatly reduced. However, this signal processing would require a large number of switches, e.g., for introducing variable delays into each of 10,000 channels. Assuming about 5,000,000 switches are required and that each dissipates about 50 .mu.W of heat, then 250 Watts would have to be dissipated by the probe. This would be excessive in the absence of some cooling system which would add further to the bulk and power requirements of the probe.
What is needed is an ultrasound imaging system which permits electronic control of both focussing and steering in two dimensions without the limitations in elevational resolution that characterize linear arrays and without the cabling and heat dissipation problems that face large two-dimensional phased array ultrasound imaging systems.